1. Field of the Invention
This invention relates to deflection of blades in a rotary machine and, more particularly, to methods and apparatus for measuring the amplitude of deflection of operating blades.
2. Description of the Prior Art
Scientists and engineers within the turbine engine field have long recognized that vibratory damage adversely limits the life of many turbine machines. They have also recognized that the blades of the rotor assembly are among the most susceptible of compressor components to vibratory damage. The blades are of necessity designed for low weight in order to minimize the centrifugallly generated loads on the rotor. Lightweight blades, however, are not always compatible with the durability requirements of the engine and may severely limit the operating life of the engine where adverse vibratory stimuli induce radical deflections in the blades.
The frequency of the adverse stimuli may be an integral multiple or a nonintegral multiple of the speed of the engine rotor is revolutions per minute (RPM). Integral vibratory stimuli are principally produced as a result of nonuniform pressure patterns upstream of the blades. As each blade is cycled from a low loading condition to a higher loading condition, the variation produced causes the blades to cyclically deflect and a strain is imposed on the blade material. One particularly distinctive form of integral vibration is known as "resonance". At resonance, the natural frequency of the installed blade is coincident with the frequency of the stimuli. The deflection amplitudes become reinforcing and the likelihood of vibratory damage is substantially increased.
Nonintegral vibration may occur at any speed and, in a most destructive mode, is referred to in the industry as "flutter". During flutter, self-excitation of the blades occurs as unsteady forces and moments created by periodic blade deflections do positive work on the blading. The periodic blade vibration may consist of blade bending or torsion or a combination of the two.
Structurally improved blade designs making judicious use of material are possible where accurate measurement of blade deflection enhances the understanding of vibratory effects. Collaterally, the measurement of airfoil deformation in response to gas pressure and centrifugal loadings enables improved aerodynamic designs.